Forsyth was born in Montréal, Québec. She was married to actor Michael Tolan on June 28, 1966 and divorced in 1975, they had one child. She later married Alan Horwitz in 1980. She became a naturalized U.S. citizen in August 1965, aged 22. Today, still married and with one child, Rosemary Forsyth continues to act. She has appeared in John Carpenter's film Ghosts Of Mars and in dozens of TV shows like Ally McBeal, NYPD Blue, Without a Trace and Monk.